1. Field of the Invention
The present invention relates to a semiconductor device including a circuit composed of field effect transistors (FET) such as thin film transistors (TFT), and to a method for manufacturing the semiconductor device. The TFT means a semiconductor element including a semiconductor layer, a gate electrode, a source electrode and a drain electrode.
Here, the “element substrate” generally indicates the substrate having a semiconductor element such as the TFT.
Here, the “display device” generally indicates the device for displaying the brightness with the change in an electric signal, and the display device for displaying by applying the electric signal to a liquid crystal.
2. Related Art
In recent years, there has been noted a technique for constructing the TFT by using a semiconductor thin film (having a thickness of about several to several hundreds nm) formed over a substrate having an insulating surface. The TFT is widely applied to an electronic device such as an IC or a semiconductor device and is urgently demanded for a development as the switching element of the liquid crystal display device.
This liquid crystal display device is coarsely divided into two known kinds: the active matrix type and the passive matrix type. The liquid crystal display device of the active matrix type uses the TFT as the switching element and can achieve an image of a high quality. The general application of the active matrix type is a note-type personal computer but is expected as a home TV set or a mobile terminal.
Of the liquid crystal display device of the active matrix type, a liquid crystal display device of a projection type is enabled to achieve a large screen display by enlarging the frame to a screen. In the projection type liquid crystal display device of recent years, there has been developed the technique for making the liquid crystal display device mobile by reducing the size of the liquid crystal panel to reduce the size of the optical system. With this size reduction in the optical system, the cost for the optical system can be lowered to provide the liquid crystal display device at a reasonable cost.
Here, it is customary to subject the active matrix type liquid crystal display device to line inversion drives. Of these line inversion drives, the source line inversion drive is made such that the polarities of signal voltages to be written in pixel TFTs connected with m-columns of signal lines are different between the adjoining signal lines, as shown in a top plan view of the pixel portion of FIG. 30. For odd frames (FIG. 30A) and even frames (FIG. 30B), moreover, there are changed the polarities of the signal voltages to be written in the pixel TFT-s connected with signal lines. The liquid crystals are prevented from burning by changing the polarities of the signal voltages to be written in the pixel TFTs thereby to drive the liquid crystal with an AC current. The gate line inversion drive may be effected by replacing the signal lines of FIG. 30 by scanning lines.
An object of the present invention is to provide an active matrix type liquid crystal display device and an element structure which is enabled to prevent an optical leakage and a disclination by examining the principle of the disclination and the optical leakage of a liquid crystal systematically.
In the interface of an alignment film, the liquid crystal is oriented to raise its one end. The “pre-tilt direction” will be termed as the positive projection of the direction, as extended from one end of the liquid crystal molecule the closest to the interface of the alignment film to the other end raised from the alignment film, upon the substrate face. Moreover, the “pre-tilt angle” will be termed as the angle made between the interface of the alignment film and the longer axis of the liquid crystal near the interface of the alignment film. The pre-tilt angle is given by a rubbing treatment or by applying an electric field to the liquid crystal to switch the liquid crystal near the interface of the alignment film.
Moreover, the orientation failure, as caused by the fact that the direction of the pre-tilt of the near liquid crystal is generally opposed in the alignment film interface, will be called the “disclination”. On the other hand, there is a region where the pre-tilt angle is locally different due to the electric field distribution or the heterogeneous rubbing although the pre-tilt direction of the liquid crystal is identical. The orientation failure, as thus caused when the orientation state is not normal, is that the brightness is locally so high when two polarization plates are arranged on the liquid crystal panel that the light looks as if it leaks. Therefore, the orientation of the liquid crystal, in which the direction of the pre-tilt is identical but the pre-tilt angle is locally different, will be called the “optical leakage”.
When the liquid crystal display device is driven by the active matrix type, the display quality is degraded by the optical leakage and the disclination. In other words, in the normally white mode, a shielding film is required for shielding the optical leakage and the disclination so that the aperture ratio decreases.
In the liquid crystal display device having fine pixels as in the projection type liquid crystal display device, the disclination and the optical leakages, if any, will take an innegligible ratio in the pixels. If the optical leakage and the disclination cannot be shielded due to the misalignment of the shielding film, moreover, they are visually recognized like bright lines, when displayed in black, to lower the contrast. In short, it is important for the projection type liquid crystal display device how the optical leakage and the disclination are to be suppressed.
As compared with the smectic liquid crystal having a layer structure and accordingly a high orientation order, the nematic liquid crystal is easily subjected to the disclination and the optical leakage by the electric field established between pixel electrodes. Especially in the orientation system using the nematic liquid crystal, therefore, countermeasures are required for reducing the disclination and the optical leakage.
The reason why the optical leakage and the disclination occur will be described with reference to a schematic diagram presenting a section of the pixel portion of a liquid crystal display device of FIG. 12. In the adjoining pixel electrodes of FIG. 12, it is assumed that a pixel electrode 101a has a potential of +5 V whereas a pixel electrode 101b has a potential of −5 V. An opposed electrode 102 is assumed to have a potential of 0 V. In a region where equipotential lines 103 are formed in parallel with the surfaces of the pixel electrodes, the positive type liquid crystals are arranged such that their molecules 108 are normal at their longer axes to the surfaces of the pixel electrodes. At the ends of the pixel electrodes, however, the equipotential lines curve so that liquid crystal molecules 106 are oriented obliquely with respect to the surfaces of the pixel electrodes to cause the orientation failure. It is considered important for reducing the orientation failure how much the curvatures of the equipotential lines at the ends of the pixel electrodes are reduced.
At the ends of the pixel electrodes, there are formed regions 104 of the optical leakage in which the pre-tilt angle is locally different. This is because the equipotential lines curve at the end portions of the pixel electrodes so that the liquid crystal molecules 106 cannot be so switched at the end portions of the pixel electrodes as to have their longer axes normal to the surfaces of the pixel electrodes.
Moreover, there is formed a region where the pre-tilt direction of the liquid crystals is reversed by the electric field at the ends of the pixel electrodes from the pre-tilt direction determined by a rubbing direction 107. Then, the pre-tilt angle and the pre-tilt direction of the alignment film interface are locally abruptly changed to enlarge the orientation strain of the liquid crystals thereby to form regions 105 where the disclination occurs.
It is understood that the disclination and the optical leakage are caused from one reason that the equipotential lines parallel to the surfaces of the pixel electrodes curve at the end portions of the pixel electrodes. The present invention to be described in the following is so structurally devised that the curvatures of the equipotential lines are suppressed as much as possible and that the equipotential lines curve, if so, as close to the ends of the pixel electrodes as possible.